Synchronisation

ABSTRACT

With OFDM systems the frequency domain data is the Fourier transform of the received time domain OFDM frames. The time domain frames must be sampled, at the receiver, in synchronism with the transmitter, so that each received frame contains data from only a single transmitted frame. It is vital for this synchronism to be maintained in order to maintain the orthogonality of the frames. A typical multi-carrier system, of the OFDM type, which uses a cyclic prefix permits orthogonality to be maintained when there is a small deviation from exact frame synchronisation. Because the signalling interval includes both an entire frame and the cyclic prefix, which is a repetition of part of the frame, a frame sampled within the signalling interval will contain data from only one frame. Since the signalling interval is greater than the frame period, this gives some leeway in frame alignment. In a multi-carrier system of the OFDM type, an adaptive channel equalizer, operating in the frequency domain, is often used. The internal parameters in such an equalizer contain, in addition to information about the characteristics of the channel, information which can be interpreted as the time deviation between the sampling clocks of the transmitter and the receiver. The present invention utilizes this information to control the sampling clock of the receiver in a more robust way than has previously been possible with known techniques. The present invention is particularly suitable for use in ADSL and VDSL modems which can be used to give broadband access over copper networks. The invention is also of relevance to broadband transmission in mobile and semi-mobile systems for transmission over the radio channels.

The present invention relates to an OFDM transmission system, an OFDMreceiver, OFDM modems including ADSL modems and VDSL modems, and methodsof synchronising an OFDM receiver with an incoming multi-carrier signal,in particular, the present invention relates to sampling clockoscillator control for an OFDM system.

In this specification the term OFDM (Orthogonal Frequency DivisionMultiplex) type is intended to include DMT (Discrete Multi-Tone).

The demand for provision of multi-media and other broad bandwidthservices over telecommunications networks has created a need to transmithigh bit rate traffic over copper pairs. This requirement has led to thedevelopment of a number of different transmission schemes, such as, ADSL(Asynchronous Digital Subscriber Line) and VDSL (Very high bit-rateDigital Subscriber Lines). One of the more likely modulation systems forall these transmission schemes is a line code known as DMT (discretemulti-tone), which bears a strong resemblance to orthogonal frequencydivision multiplex, and is a spread spectrum transmission technique.

In discrete multi-tone transmission, the available bandwidth is dividedinto a plurality of sub-channels each with a small bandwidth, 4 kHzperhaps. Traffic is allocated to the different sub-channels independence on noise power and transmission loss in each sub-channel.Each channel carries multi-level pulses capable of representing up to 11data bits. Poor quality channels carry fewer bits, or may be completelyshut down.

Because inter pair interference in copper pair cables is higher wheredata is transmitted in both directions, i.e. symmetric duplex, a numberof transmission schemes have proposed the use of asymmetric schemes inwhich high data rates are transmitted in one direction only. Suchschemes meet many of the demands for high bandwidth services, such as,video-on-demand.

VDSL technology resembles ADSL to a large degree, although ADSL mustcater for much larger dynamic ranges and is considerably more complex asa result. VDSL is lower in cost and lower in power, and premises VDSLunits need to implement a physical layer media access control formultiplexing upstream data.

Four line codes have been proposed for VDSL:

CAP; Carrierless AM/PM, a version of suppressed carrier QAM, for passiveNT configurations, CAP would use QPSK upstream and a type of TDMA formultiplexing (although CAP does not preclude an FDM approach to upstreammultiplexing);

DMT; Discrete Multi-Tone, a multi-carrier system using Discrete FourierTransforms to create and demodulate individual carriers, for passive NTconfigurations; DMT would use FDM for upstream multiplexing (althoughDMT does not preclude a TDMA multiplexing strategy);

DWMT; Discrete Wavelet Multi-Tone, a multi-carrier system using WaveletTransforms to create and demodulate individual carriers, DWMT also usesFDM for upstream multiplexing, but also allows TDMA; and

SLC; Simple Line Code, a version of four-level baseband signalling thatfilters the base band and restores it at the receiver, for passive NTconfigurations; SLC would most likely use TDMA for upstreammultiplexing, although FDM is possible.

Early versions of VDSL will use frequency division multiplexing toseparate downstream from upstream channels and both of them from POTSand ISDN. Echo cancellation may be required for later generation systemsfeaturing symmetric data rates. A rather substantial distance, infrequency, will be maintained between the lowest data channel and POTSto enable very simple and cost effective POTS splitters. Normal practicewould locate the downstream channel above the upstream channel. However,the DAVIC specification reverses this order to enable premisesdistribution of VDSL signals over coaxial cable systems.

In a multi-carrier system, such as a DMT system, a receiver must be ableto recover a sampling clock that is very accurately synchronized to atransmitter sampling clock. A known method, for achievingsynchronization, uses a reserved carrier, the pilot carrier, which istransmitted with a fixed phase. The receiver sampling clock is thenphase locked to the pilot carrier. Frame timing must also be recovered.In existing systems this has been achieved by using a correlationtechnique operating in the time domain.

With OFDM systems the frequency domain data is the Fourier transform ofthe received time domain OFDM frames. The time domain frames must besampled, at the receiver, in synchronism with the transmitter, so thateach received frame contains data from only a single transmitted frame.It is vital for this synchronism to be maintained in order to maintainthe orthogonality of the frames.

A typical multi-carrier system, of the OFDM type, which uses a cyclicprefix, permits orthogonality to be maintained when there is a smalldeviation from exact frame synchronisation. Because the signallinginterval includes both an entire frame and the cyclic prefix, which is arepetition of part of the frame, a frame sampled within the signallinginterval will contain data from only one frame. Since the signallinginterval is greater than the frame period, this gives some leeway inframe alignment.

In a multi-carrier system of the OFDM type, the control of the receiversampling clock is critical for achieving an optimal utilization of thechannel capacity. The present invention takes the data needed for thiscontrol function from the received signal in a novel manner, inparticular, the present invention uses adaptive equalizer parameters forsampling clock oscillator control.

In a multi-carrier system of the OFDM type, an adaptive channelequalizer, operating in the frequency domain, is often used. Theinternal parameters in such an equalizer contain, in addition toinformation about the characteristics of the channel, information whichcan be interpreted as the time deviation between the sampling clocks ofthe transmitter and the receiver. The present invention utilizes thisinformation to control the sampling clock of the receiver in a morerobust way than has previously been possible with known techniques.

Known techniques for achieving frame synchronisation do not operateentirely in the frequency domain. A technique for frame synchronisationin which only frequency domain data is employed, is described in ourco-pending patent application Kgp 74/97.

The present invention is particularly suitable for use in ADSL and VDSLmodems which can be used to give broadband access over copper networkswith relatively stationary channels. The invention is, however, ofgeneral application and also of relevance to broadband transmission inmobile and semi-mobile systems for transmission over the radio channels.

The present invention provides an extremely robust estimation of thetime deviation between the sampling clocks of the transmitter and thereceiver and can handle deviations of several periods, which impliesthat symbol limits are also guided to the right location. The robustnessis achieved by using all active carriers in the estimation.

According to a first aspect of the present invention, there is provideda receiver, for use in an OFDM type transmission system, in which datais transmitted in frames, each frame having a cyclic prefix which is arepetition of part of said frame, characterised in that control meansare provided which control a sampling oscillator, and in that saidcontrol means include estimation means for estimating timing deviationsof said sampling clock, said estimation means operating entirely onfrequency domain input data.

According to a second aspect of the present invention, there is provideda receiver, for use in an OFDM type transmission system, in which datais transmitted in frames, each frame having a cyclic prefix which is arepetition of part of said frame, and in which said receiver has anadaptive equaliser having an equaliser inverse channel model,characterised in that separation means are provided for separating saidequaliser inverse channel model into a first and a second part, saidfirst part being independent of sample timing and said second part beingdependent on sample timing and in that control means are provided whichcontrol a sampling oscillator in dependence on said second part.

Said control means may include estimation means for estimating timingdeviations of said sampling clock, said estimation means operatingentirely on frequency domain input data.

Said estimation means may estimate an approximation of a linear portionof an argument function produced by timing deviations of said samplingoscillator.

Said estimation means may be adapted to find a linear part of saidargument function by taking an average slope of said argument function.

Said approximation of a linear portion of an argument function may beused as a feedback control signal for said sampling clock.

Said approximation of a linear portion of an argument function may havea slope which converges to zero as a control loop, for said samplingclock, settles.

Those parts of said equaliser inverse channel model, other than saidlinear portion of said argument function, may be controlled by saidequaliser, which continuously adapts to variations in sampling timing.

Said equaliser and said control means may each use well defined anddifferent portions of said equaliser inverse channel model to achieve anoutput frequency domain signal with zero phase deviation relative to atransmitted signal.

Said slope of said argument function, as, may be estimated from theequation$\alpha_{k} = {\frac{1}{N}{\sum\limits_{n}{L\frac{X_{n,k}}{n}}}}$

where N is the number of active carriers and X_(n,k) is the unwrappedargument function for the nth carrier in the kth frame.

Said slope of said argument function, α_(k), may be estimated from theequation$\alpha_{k} = {\frac{2}{n_{2} - n_{0}}\left\lbrack {{\sum\limits_{n = {n_{1} + 1}}^{n_{2}}{LX}_{n,k}} - {\sum\limits_{n = n_{0}}^{n_{1}}{LX}_{n,k}}} \right\rbrack}$

where N is the number of active carriers, X_(n,k) is the unwrappedargument function for the nth active carrier in the kth frame, indicesn₀ and n₂ are the lower and upper limits respectively of the band andindex n₁ divides the band into two equal parts.

On start up, frame timing may be adjusted until received frames aresampled inside a signal interval.

Means may be provided, responsive to a feed back control for saidsampling oscillator, to adjust said frame timing so that framesynchronization is maintained.

According to a third aspect of the present invention, there is an OFDMtype transmission system in which data is transmitted in frames, eachframe having a cyclic prefix which is a repetition of part of saidframe, characterised in that said system includes a receiver as definedin any preceding paragraph.

According to a fourth aspect of the present invention, there isprovided, in an OFDM type system in which data is transmitted in frames,each frame having a cyclic prefix which is a repetition of part of saidframe, a method of synchronising a receiver sampling oscillator with atransmitter sampling oscillator, characterised by controlling saidsampling oscillator with a feedback signal representing an estimation oftiming deviations of said sampling clock, said estimation signal deriveddirectly from domain input data.

According to a fifth aspect of the present invention, there is provided,an OFDM type system in which data is transmitted in frames, each framehaving a cyclic prefix which is a repetition of part of said frame, andin which said receiver has an adaptive equaliser having an equaliserinverse channel model, a method of synchronising a receiver samplingoscillator with a transmitter sampling oscillator, characterised byseparating said equaliser inverse channel model into a first and asecond part, said first part being independent of sample timing and saidsecond part being dependent on sample timing and controlling a samplingoscillator in dependence on said second part.

Timing deviations of said sampling clock may be estimated entirely fromfrequency domain input data.

An approximation of a linear portion of an argument function produced bytiming deviations of said sampling oscillator may be estimated.

A linear part of said argument function may be found by taking anaverage slope of said argument function.

Said approximation of a linear portion of an argument function may beused as a feedback control signal for said sampling clock.

Said approximation of a linear portion of an argument function may havea slope which converges to zero as a control loop, for said samplingclock, settles.

Those parts of said equaliser inverse channel model, other than saidlinear portion of said argument function, may be controlled with saidequaliser, which continuously adapts to variations in sampling timing.

Said equaliser and said control means may each use well defined anddifferent portions of said equaliser inverse channel model to achieve anoutput frequency domain signal with zero phase deviation relative to atransmitted signal.

Said slope of said argument function, α_(k), may be estimated from theequation$\alpha_{k} = {\frac{1}{N}{\sum\limits_{n}{L\frac{X_{n,k}}{n}}}}$

where N is the number of active carriers and X_(n,k) is the unwrappedargument function for the nth carrier in the kth frame.

Said slope of said argument function, α_(k), may be estimated from theequation$\alpha_{k} = {\frac{2}{n_{2} - n_{0}}\left\lbrack {{\sum\limits_{n = {n_{1} + 1}}^{n_{2}}{LX}_{n,k}} - {\sum\limits_{n = n_{0}}^{n_{1}}{LX}_{n,k}}} \right\rbrack}$

where N is the number of active carriers, X_(n,k) is the unwrappedargument function for the nth active carrier in the kth frame, indicesn₀ and n₂ are the lower and upper limits respectively of the band andindex n₁ divides the band into two equal parts.

Frame timing, on start up, may be adjusted until received frames aresampled inside a signal interval.

Said frame timing may be adjusted in accordance with a feed back signalfor said sampling oscillator, so that frame synchronization ismaintained.

According to a sixth fifth aspect of the present invention, there isprovided an ADSL modem characterised in that said modem has a receiveras defined above, or operates a method of synchronisation as definedabove.

According to a sixth aspect ofthe present invention, there is provided aVDSL modem characterised in that said modem has a receiver as definedabove, or operates a method of synchronisation as defined above.

Embodiments of the invention will now be described, by way of example,with reference to the accompanying drawings, in which:

FIG. 1 illustrates, in functional form, an equaliser and samplingcontrol unit in which the present invention can be implemented.

FIG. 2 illustrates the time domain data format of an OFDM signal usedwith the present invention.

The synchronisation process of the present invention is illustrated in afunctional form in FIG. 1. Incoming frequency domain data is passed viaan equaliser to a detector/quantizer and thence to a symbol decoder. Theoperation of the remaining blocks shown in FIG. 1, namely theequalisation parameter updating algorithm, the sampling clock controlalgorithm and the frame timing algorithm are explained in the followingdescription. It is, however, worth noting at this point that:

the equalisation parameter updating algorithm takes inputs from the rawfrequency domain input data, X, the output of the equaliser, U, and theoutput of the detector/quantizer, Y;

the sampling clock control algorithm receives an input from theequalisation parameter updating algorithm, as does the equaliser; and

the frame timing algorithm accepts an input from the raw frequencydomain input data.

The frequency-domain data comprises the received time-domain OFDM framesafter Fourier transformation. The time-domain frames must be sampled insynchronism with the transmitter so that each received frame containsdata from only one transmitted frame. This is important in order tomaintain the orthogonality of the frames.

FIG. 2 shows the time-domain format for the transmission of OFDM framesused with the present invention.

The signalling interval contains a cyclic prefix and a frame. The cyclicprefix is a copy of the last part of the frame. This means that a framesampled anywhere inside the signalling interval will contain data fromone transmitted frame only. A deviation from the exact frame timingwill, therefore, lead to a cyclic permutation of the frame. Theorthogonality will, however, be maintained.

As previously stated, the present invention relates to, among otherthings, a method for sampling clock oscillator control in a system ofthe OFDM type, which is based on adaptive equalizer information. Themethod of the present invention assumes that the linear part of theequalizer parameter argument vector is related to the frame timingdeviation. The estimation of the frame timing deviation is performedentirely in the frequency domain and the deviation estimate is used as afeed-back control signal for the sampling clock Oscillator.

A training procedure must be used at start-up. The frame timing isadjusted until the received frames are sampled inside the signallinginterval, see our co-pending patent application Kgp 74/97. The samplingclock frequency must also be adjusted so that it is sufficiently closeto the transmitter clock frequency to enable the equalizer to followchanges in the timing deviation.

Frame start pulses are generated by counting sampling clock intervals.

Therefore, after the initial setting of the frame start pulse timingduring the training procedure, the timing of the frame start pulse willneed to be modified by the feed-back control of the sampling clockoscillator, in order to maintain frame synchronisation.

After the training procedure, the equalizer parameters EQ will representthe complex frequency domain inverse of the channel. If there is adeviation from the correct timing of the time domain sampling of theframes, there will also be a linear part of the equalizer inversechannel model argument function. The adaptive equaliser constructs amodel of the transmission channel and applies an inverse of this modelto incoming signals—the equaliser inverse channel model.

The exact linear argument function, produced by the timing deviation, isnot available, but an approximation can be estimated using the equalizerparameters. The argument function of the equalizer parameters isgenerally non-linear, but a linear part can be found by taking theaverage slope of the argument function. This slope estimate is used as afeed-back signal to control the sampling clock oscillator frequency. Theslope will converge towards zero as the sampling clock control loopsettles.

The rest of the equaliser inverse channel model is taken care of by theequalizer, which continuously adapts to variations in the sample timing.

The advantage of this technique is that the equalizer and the samplingcontrol use well defined separate parts of the equaliser inverse channelmodel to achieve an output frequency domain signal with zero phasedeviation relative to the transmitted signal.

The argument function of the equalizer parameters is the vector ofarguments of the individual complex elements. The argument of a complexnumber is the inverse tangent of the imaginary part divided by the realpart. A problem associated with this calculation is that the inversetangent function is periodic, with a period 2 π radians. In thisapplication it is necessary to handle larger arguments than π radians,which is the range of the inverse tangent function. An assumption usedhere is that the difference in argument between adjacent parameters issmaller than n radians. This means that it is possible to compensate foreach discontinuity caused by the inverse tangent function periodicityand thus unwrap the argument function.

The average slope, α_(k), of the linear part of the argument functioncan be calculated, as shown in equation (1), or by some other standardmethod, using the unwrapped argument function of X_(k) for the kth frame$\begin{matrix}{\alpha_{k} = {\frac{1}{N}{\sum\limits_{n}{L\frac{X_{n,k}}{n}}}}} & (1)\end{matrix}$

where N is the number of active carriers and X_(n,k) is the unwrappedargument function for the nth carrier in the kth frame.

If the lowest frequency carriers are not present in the frame, it is notpossible to find the true argument function, because there will be anunknown starting value for the available part of the function. This isnot a problem in the present case, since the slope can still becalculated.

Equation (2) shows an algorithm that gives the average slope of acontiguous band of active carriers. Indices n₀ and n₂ are the lower andupper limits respectively of the band. Index n₁ divides the band intotwo equal parts. If several separate bands are used, equation (2) isapplied to each band and the average of the results is calculated.$\alpha_{k} = {\frac{2}{n_{2} - n_{0}}\left\lbrack {{\sum\limits_{n = {n_{1} + 1}}^{n_{2}}{LX}_{n,k}} - {\sum\limits_{n = n_{0}}^{n_{1}}{LX}_{n,k}}} \right\rbrack}$

The algorithm, according to equation (2), gives a very simple hardwareimplementation for OFDM receivers.

The unique novelty in the technique of the present invention resides inthe separation of the inverse channel model into two parts, one of whichis sample timing dependent and the other of which is sample timingindependent.

If the sample timing and the equalizer are controlled by separatetechniques they might counteract each others actions, because, both theequalizer and the sample timing influence time delay. This situationcould eventually lead to a drift of the frame timing out of the correctinterval (the cyclically permuted signal interval). This cannot happenwith the technique of the present invention.

The sample timing control, provided by the present invention, is veryrobust against external disturbance, because every active carrier isused in the timing deviation estimation.

What is claimed is:
 1. A receiver, for use in an OFDM transmission system in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of the frame, the receiver comprising: a sampling oscillator; an adaptive equalizer having an equalizer inverse channel model; separation means for separating the equalizer inverse channel model into a first and a second part, the first part being independent of sample timing and the second part being dependent on sample timing; and control means for controlling the sampling oscillator based upon the second part.
 2. A receiver according to claim 1 wherein said control means comprises estimation means for estimating timing deviations of said sampling oscillator; and wherein said estimation means operates entirely on frequency domain input data.
 3. A receiver according to claim 2 wherein said estimation means estimate an approximation of a linear portion of an argument function produced by timing deviations of said sampling oscillator.
 4. A receiver according to claim 2 wherein said estimation means finds the linear portion of the argument function by taking an average slope of the argument function.
 5. A receiver according to claim 4 wherein the approximation of the linear portion of the argument function is used as a feedback control signal for said sampling oscillator.
 6. A receiver according to claim 5 further comprising a control loop for said sampling oscillator; and wherein the approximation of the linear portion of the argument function has a slope which converges to zero as the control loop settles.
 7. A receiver according to claim 6 wherein parts of the equalizer inverse channel model, other than the linear portion of the argument function, are controlled by said adaptive equalizer which continuously adapts to variations in sampling timing.
 8. A receiver according to claim 7 wherein said adaptive equalizer and said control means each use defined and different portions of the equalizer inverse channel model to achieve an output frequency domain signal with zero phase deviation relative to a transmitted signal.
 9. A receiver according to claim 6 wherein the slope of the argument function α_(k) is estimated from an equation $\alpha_{k} = {\frac{1}{N}{\sum\limits_{n}{L\frac{X_{n,k}}{n}}}}$

where N is the number of active carriers and X_(n,k) is the unwrapped argument function for an nth active carrier in a kth frame.
 10. A receiver according to claim 6 wherein the slope of the argument function α_(k) is estimated from an equation $\alpha_{k} = {\frac{2}{n_{2} - n_{0}}\left\lbrack {{\sum\limits_{n = {n_{1} + 1}}^{n_{2}}{LX}_{n,k}} - {\sum\limits_{n = n_{0}}^{n_{1}}{LX}_{n,k}}} \right\rbrack}$

where N is the number of active Carriers, X_(n,k) is the unwrapped argument function for an nth active carrier in a kth frame, indices n₀ and n₂ are lower and upper limits respectively of a band and index n_(i) which divides the band into two equal parts.
 11. An OFDM transmission system in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of the frame, the OFDM transmission system comprising: a receiver comprising a sampling oscillator, an adaptive equalizer having an equalizer inverse channel model, a separation circuit for separating the equalizer inverse channel model into a first and a second part, the first part being independent of sample timing and the second part being dependent on sample timing, and a controller for controlling the sampling oscillator in dependence on the second part.
 12. In an OFDM system in which data is transmitted in frames, each frame having a cyclic prefix which is a repetition of part of the frame, and in which the receiver comprises an adaptive equalizer having an equalizer inverse channel model, a method of synchronizing a receiver sampling oscillator with a transmitter sampling oscillator, the method comprising: separating the equalizer inverse channel model into a first and a second part, the first part being independent of sample timing and the second part being dependent on sample timing; and controlling a sampling oscillator based upon the second part.
 13. A method according to claim 12 further comprising estimating timing deviations of the receiver sampling oscillator entirely from frequency domain input data.
 14. A method according to claim 13 wherein estimating comprises estimating an approximation of a linear portion of an argument function produced by timing deviations of the receiver sampling oscillator.
 15. A method according to claim 14 wherein estimating an approximation of a linear portion of an argument function comprises taking an average slope of the argument function.
 16. A method according to claim 14 further comprising using the approximation of a linear portion of an argument function as a feedback control signal for the receiver sampling oscillator.
 17. A method according to claim 16 wherein the approximation of a linear portion of an argument function has a slope which converges to zero as a control loop for the receiver sampling oscillator settles.
 18. A method according to claim 17 further comprising controlling parts of the equalizer inverse channel model, other than the linear portion of the argument function, with the adaptive equalizer which continuously adapts to variations in sampling timing.
 19. A method according to claim 18 wherein the adaptive equalizer and the control loop each use defined and different portions of the equalizer inverse channel model to achieve an output frequency domain signal with zero phase deviation relative to a transmitted signal.
 20. A method according to claim 17 wherein estimating the slope of the argument α_(k) uses an $\alpha_{k} = {\frac{1}{N}{\sum\limits_{n}{L\frac{X_{n,k}}{n}}}}$

where N is the number of active carriers and X_(n,k) is the unwrapped argument function for an nth active carrier in a kth frame.
 21. A method according to claim 17 wherein estimating the slope of the argument function α_(k) uses an equation $\alpha_{k} = {\frac{2}{n_{2} - n_{0}}\left\lbrack {{\sum\limits_{n = {n_{1} + 1}}^{n_{2}}{LX}_{n,k}} - {\sum\limits_{n = n_{0}}^{n_{1}}{LX}_{n,k}}} \right\rbrack}$

where N is the number of active carriers, X_(n,k) is the unwrapped argument function for an nth active carrier in a kth frame, indices n₀ and n₂ are lower and upper limits respectively of a band and index n_(i) which divides the band into two equal parts.
 22. A method according to claim 21 further comprising adjusting frame timing, upon starting, until received frames are sampled within a signal interval.
 23. A method according to claim 22 wherein adjusting the frame timing comprises adjusting the frame timing in accordance with a feed back signal so that the sampling oscillator maintains frame synchronization. 